Those are all good quality, but I must say that Brownells has the best screwdrivers made, Have some that I use everyday on the job now for 20 years, never one problem with them they are the magna tip drivers, check'em out they are worth the money!

While steel type and hardness factors into it, the primary difference is that gunsmithing screwdrivers have hollow-ground tips. The contractor/handyman/household screwdrivers (Craftsman, etc.) are taper-ground. They're great screwdrivers around the house, but they are not suited to gunsmithing.
If you look at the profile of a hollow-ground screwdriver, the blade is the same width for 1/8 inch or so back from the tip, then flares out rapidly to the width of the shank. A taper-ground blade has a profile that tapers from the tip to the width of the shank.
Gun screws tend to have fairly narrow, deep slots, because they frequently get torqued more than typical houshold screws. Unlike taper-ground screwdrivers, a hollow-ground screwdriver tip will fill the entire slot of the screw and not try to "walk" out when you torque it.

Bottom line: Those buggered up screwheads you see on older guns come from using taper ground screwdrivers.

Hahaha, come on guys, gunsmith screwdrivers are not tapered like a wedge, they are tapered to a certain point that flate and straight to the tip, and the shaft is the same size as the tip for strength, if you use the one from a standard tool kit, you'll see that they are tapered to the tip, there fore the slot on the bolt will only contact the top of the bolt head slot, with a wedged screwdriver, you will buger it up because the driver will go up when you trun it and the gunsmith screwdriver will stay in the slot, because the driver is straight not tapered, and with the proper screw driver and the proper head size, the bolt head will look new no matter how much times you take the bolt out and in again

On the Brownells Magna tip screw driver sets, what sizes are most commonly used on guns. They come in many different thicknesses and I want to buy a set but don't want to have three that get used all the time and nine that stay in the original wrapper for a year.

Depends on your gun(s). If you don't own any K-98 Mausers, for example, the bit for Mauser action screws is likely to go unused for a very long time.
Yes, you'll definitely put a lot of wear on a few bits, and almost none on some others. For me, the smaller bits get used a lot, the big ones less often, and the "middle" sized ones least. You'll figure out quickly what your high-use bits are. Buy extras of those: when you lose or break that bit, it's nice to have the spare already on hand.
The idea of buying only what you'll use the most may seem efficient, but it's false economy. When you do need that one oddball bit, you won't have it, and you'll use the next closest bit, or <gasp!> a taper-ground screwdriver. Either way, you have a better than 50/50 chance of buggering up the slot, which is what you're trying to avoid in the first place.

The Brownells or Chapman basic sets are in the $30-50 range, and are well worth it.
While you're at it, get a set of punches, both flat-faced and roll-pin. Craftsman punches are nice: if you use them a lot, eventually you'll bend or break one, and Sears still replaces them free.

I have always just used the Dremel and the cutoff disks to shape regular screwdriver blades to properly fit whatever screw slots I had need of fitting. I aint got $60 to waste on a specialized set of screwdriver bits. I made/make almost all the other tools needed for smithing, too. Ferchrissakes, man, who does enough barrel removals to justify tyinig up $300 in a barrel vise and action wrench? I made mine in 3 hours. $100 an hour, tax free, is pretty good pay, in my book! :-)